Beer volatile analysis relates to the analysis of volatiles dissolved in beer. Beer volatile analysis is sometimes referred to as beer headspace analysis, but will be referred to as beer volatile analysis herein. In volatile analysis, volatiles are isolated and typically analyzed in the gaseous phase using gas chromatography. There are other methods for determining the composition of the isolated volatiles (such as nondispersive infrared absorption, or laser techniques), but it is common to use gas chromatography. Volatile analysis can accurately identify the amount and types of volatile components in beer and, for this reason, it is useful for analyzing beer. In particular, volatile analysis is useful for testing beer flavor.
Because the concentration of volatiles in a sample are extremely low, the volatiles must typically be concentrated prior to chromatographic analysis. For volatile analysis to be accurate and sensitive, it is important to preserve the integrity of the volatiles as the volatiles are isolated, concentrated, and transported for chromatographic analysis. Heretofore, most volatile analysis systems have used "purge and trap" methods to isolate the volatiles from the beer (or other aqueous liquid). That is, a purging flow of pure helium gas is bubbled through beer and beer volatiles are trapped and concentrated on a porous polymer trap. The porous polymer trap is then transported to volatile analysis equipment where the trap is heated so that beer volatiles release or desorb from the trap. The desorbed beer volatiles are then carried into a gas chromatograph by flowing helium (or some other carrier gas) through the heated trap and into the gas chromatograph.
As mentioned above, the composition of the volatiles is usually measured using gas chromatography (GC). In capillary column gas chromatography, a sample of volatiles is injected into a capillary that has a thin film of methylsiloxane or some other liquid phase in which the different volatiles in the sample have different solubility. This is the primary mechanism for separating the compounds in the mixture. The capillary is then flushed with an inert carrier gas (e.g. helium) that transports the volatiles at different rates to a detector at the end of the capillary. Each volatile in the sample can be identified by its retention time in the capillary.
Capillary column gas chromatography is preferred over packed column gas chromatography because present capillary column GC can give better resolution than packed column GC. However, capillary columns allow only very small flow rates. This presents a problem because the desorption flow rate of helium needed to carry desorbed volatiles from a heated polymer trap into a capillary column is typically higher than that allowed through capillary columns. This problem is overcome in some systems by venting most of the helium gas that is carrying volatiles desorbed from the heated polymer trap to the atmosphere, and routing only a portion to the capillary column. In these systems, trace components are sometimes confused with base line noise in chromatographic data; or even lost entirely to the atmosphere.
FIGS. 1a and 1b show a prior art system where volatiles are transported from a heated polymer trap 15 to a cold trap 10 without venting a portion of the desorption helium gas to the atmosphere before cold trapping the volatiles in the cold trap 10 (see desorption/cold trapping mode shown in FIG. 1a). After volatiles are trapped in the cold trap 10, the cold trap 10 is heated and the volatiles are injected into a capillary column 21 located in a GC oven 14 by helium flowing at a relatively slow analysis flow rate (see injection mode shown in FIG. 1b).
In this prior art system, the cold trap 10 is within a capillary interface 12 located outside of a GC oven 14. The capillary interface 12 is a modified Model 1000 Capillary Interface Instrument (Tekmar, Inc., Cincinnati, Ohio) that is controlled electrically by a control unit 24. The cold trap 10 in the capillary interface 12 is a DB-1 fused silica capillary column (J&W Scientific, Inc., Folsom, Calif.) with a 0.32 mm I.D. that lies inside of a 1/16" O.D. stainless steel tube (not shown) in a liquid nitrogen reservoir (not shown). The cold trap 10 is about 5 cm in length.
Referring to FIG. 1a, a polymer trap 15, containing trapped beer volatiles, is connected to the capillary interface 12. Liquid nitrogen, supplied from a tank (not shown) is used to cool the cold trap 10 and a trap heater 16 heats the polymer trap 15 to desorb the beer volatiles from the polymer trap 15. Helium, supplied through desorption flow line 1, is then flowed through the system at a desorption flow rate which is relatively high. Desorbed beer volatiles are transported from the heated polymer trap 15 and are trapped in the cooled cold trap 10 and helium vents to the atmosphere 26. A maintenance flow 3 of helium is flowed into the capillary column 21 to maintain the integrity of the column 21.
Referring to FIG. 1b, after the volatiles are trapped in the cold trap 10, a valve 20 in the capillary interface 12 switches from a venting position to the GC capillary column 21. Then, the polymer trap 15 is removed, the carrier flow line 2 is attached to the interface 12, the cold trap 10 is heated rapidly, and helium is flowed through the system at the relatively slow analysis flow rate to inject the volatiles into the capillary column 21. The cold trap 10 can be heated rapidly because the stainless steel tubing (not shown) in the liquid nitrogen reservoir (not shown) acts as an electrical resistance heater. As the helium flow injects volatiles into thee capillary column 21, highly volatile compounds are retrapped at the head of the column 21 in the liquid phase until they are released by elevated GC oven temperatures.
In the prior art system shown in FIGS. 1a and 1b, some volatiles released from the heated polymer trap 15 might not be trapped in the cold trap 10 and, therefore, might flow out the vent 26 with the desorption flow of helium. Also, the integrity of volatile composition may be compromised by the time the volatiles are analyzed because of the several steps required to transport the volatiles to the GC column 21 (i.e. polymer trapping, desorption, cold trapping, heating, and retrapping in the liquid phase).